1. Field of the Invention
The present invention relates to a distributed power system and, more particularly, to a distributed power system used in a power transformer having a narrower normal direct current (DC) input range and keeping the output voltage thereof when the input voltage drops occasionally in a short time.
2. Description of the Prior Art
As shown in FIG. 1, a conventional distributed power system includes a power factor correction (PFC) transformer 1a, a front end (FE) DC transformer 2a, and a load end transformer 3a. The PFC transformer 1a is used to transform an alternating current (AC) input voltage into a high DC voltage (generally 400V), and performs the corresponding PFC in the input terminal thereof. The FE DC transformer 2a is used to transform the above high DC voltage into another DC voltage (generally 48V or 12V), and sends the DC voltage to the load end transformer 3a, which transforms the DC voltage into the required voltage for loads.
Above FE DC transformer 2a is configured to operate with a smaller duty ratio for the purpose of keeping the output voltage thereof constant during the sustaining time (e.g., 20 ms). That is, the FE DC transformer 2a still operates within the regulation range so that the storage capacitor of the PFC transformer 1a discharges to drop its output voltage from 400V to 300V.
However, with this smaller duty ratio, the efficiency and power density of the conventional distributed system and the energy use efficiency of the storage capacitor of the PFC transformer 1a are relatively lower.
As shown in FIG. 2, there is an asymmetric half bridge (AHB) circuit topology adopting an FE DC transformer with asymmetric windings. Under the normal working status, the duty cycle of the FE DC transformer 2a can be increased, and the output voltage of the FE DC transformer 2a can be kept constant when the output voltage of the PFC transformer 1a drops from 400V to 300V.
For the AHB transformer, the duty ratio thereof varies from 0 to 0.5 theoretically. The smaller the duty ratio is, the more “asymmetric” the operating status of the transformer will be, leading to the more imbalanced component stress and the lower efficiency. Therefore, the ideal operating status is with the duty ratio close to 0.5.
Under the situation of constant output voltage, the higher the input voltage of the AHB transformer, the smaller the duty cycle. Therefore, the output voltage of the AHB transformer can be kept constant without any processing during the sustaining time if the AHB transformer operates within the regulation range when the storage capacitor discharges and the output voltage of the PFC transformer 1a drops from 400V to 300V. In other words, the AHB transformer has a largest duty ratio when the input voltage is 300V. However, this will result in a very small duty ratio so that the operating efficiency of the AHB transformer under the normal operating status with an input voltage of 400V is inferior.
Moreover, the output ripple of the AHB transformer will increase, the power distribution of complementary loops is more imbalanced, and the energy usage efficiency of the storage capacitor of the PFC transformer 1a is lower.
As shown in FIG. 3, there is an asymmetric half bridge (AHB) transformer adopting an FE DC transformer with the range (adjustable) winding. A switch is used to change the range winding of the transformer so as to increase the duty ratio of the FE DC transformer 2a under the normal operating status when the input voltage drops from 400V to 300V.
However, during the sustaining time, if there is an abrupt change of the status of the AHB transformer, the output voltage thereof easily fluctuates. Moreover, the energy usage efficiency of the storage capacitor of the PFC transformer 1a is low.
Accordingly, the present invention aims to provide a distributed power system to resolve the problems in the prior art.